Many wireless environments include heterogeneous access technologies and networks. For example, a mobile wireless device may be able to connect to a cellular base station, a wireless local area network (WLAN), a wireless wide area network (WWAN), a wireless metropolitan area network (WMAN) and others. As mobile stations move about, they may handover communications from one connection type to another connection type. For example, a mobile device may handover from a cellular base station to a wireless local area network access point. A handover between different connection types is referred to herein as a “vertical handover.” This is in contrast to a “horizontal handover” in which the connection type remains the same. A horizontal handover may occur when a mobile device hands off from one cellular base station to another, or disassociates from one WLAN access point and associates with another.
Problems and challenges presented by vertical handovers are, in some cases, more difficult than those presented by horizontal handovers. Vertical handovers may involve two very different air interfaces (WLAN, WWAN, cellular) that make comparison of radio metrics (e.g. signal strength) difficult. Further, mobile devices and heterogeneous networks may not have ways to share inter-radio inter-network information such as available networks in a particular area (neighbour maps) to reduce the scan time and handover delays.
Many known handover schemes are based just on Received Signal Strength Indicator (RSSI), and in many cases handover decisions are made unilaterally by the mobile device. In multiple network environments, this approach may suffer from a high failure rate. For example, the RSSI of a WLAN access point may indicate a high received signal strength at the mobile device even in the presence of high interference thus leading to a high packet loss rate. In this scenario, the decision to handover to the WLAN access point might easily cause a connection failure (e.g., a dropped call) as the high-interfered WLAN radio would result in high packet loss rate.